Regeneration of molecular sieve adsorbents



March 26, 1968 T R. c. HOKE REGENERATION OF MOLECULAR SIEVE ADSORBENTSOriginal Filed Aug. 26, 1958 SIEVATE oEsoRsATi-i SIEVATE (NORMAL.PARAFFIN FR 4, DESORBATE NORMAL PARAFFINS IN VE/V TOR Rona/d 6. Hokemm. m

' ATTORNEY United States Patent Ofiice 3,375,204 Patented Mar. 26, 19683,375,204 REGENERATION OF MOLECULAR SIEVE ADSORBENTS' Ronald C. Hoke,Berkeley Heights, N.J., assignor to Esso scribed in U.S. 2,442,191.Zeolites vary somewhat in composition, but generally contain silica,aluminum, oxygen, and an alkali and/ or alkaline earth element, e.g.,sodium and/or calcium, magnesium, etc. Analcite has the empiricalformula NaAlSi O -H O. Bari'er (US. Patent No.

o l o 32323; e Engmeermg Company a corporation of 2,306,610) teachesthat all or part of the sodium is re- Continuation of application Ser.No. 757,367, Aug. 26, placeable by e to yleld on dehydrat'lonf moleeu'1958. This application Feb. 12, 1963', Ser. No. 259,773 S16v6 havlng theformula, z) z i izzq- 12 c i 1, 252 419 Black (U.S. Patent No.2,522,426) describes a synthetic molecular sieve having the formula4CaO-Al O -4SiO The synthesis of molecular sieves having uniform poreABSTRACT OF THE DISCLOSURE sizes of 4" and 5 Angstrom units ma beaccomplished by ,I I 1 I f y 1: a1 '1- Molecular sieve adsorbents areregenerated by burning' ll ian equeous an a 1 met sncate carbonaceousdeposits via a three step process involving il P m i e j to 1 heating,treating with controlled oxygen stream and conor i a ig g avmg a tactingwith air, While controlling water concentration to ratio 0 a2 t0 2 Z 0 aout at a tempera" below detrimental leveL ture of from about 160 toabout 2 15 F. in such proporti-ons as to give a ratio of Si0 to A1 03 inthe mixture of 0.5-3/1. The mixture is held at the stated temperaturesThis application is a continuation of copending applica- 0 r W 9? of F Qto form a erystalhne,sodlum tion 757 367 filed Aug 26 1958 and now abanalnrnino-silieate, which we molecular sieve material havdoned ing auniform pore size of about 4 Angstrom units. A The present inventionrelates to an improved process e Slze f about 5 Angstrom units m fprodlylcedi for the efiicient and economical separation and segregathismatenal by base reaetlon Wlbh alkalme tion of straight chain or aromatichydrocarbons from f metal Such as 9 the form of calclum ch10- rriixturescomprising straight chain hydrocarbons, branched e e example i etherInstance? the meleeular Sieve hydrocarbons, cyclic naphthenes andaromatics. The ini i produced is water Washed and 'acnvated by ventionis particularly concerned with a method of imth f 1 U1 1 h I proving theoverall thermal efficiency of a molecular sieve g e 1 $2 ee ar eleve e gseparation process including both the adsorptionv stage 30 e O a out e Fg eej and the desorption stage, and of periodically restoring the i anequeous e' 0 1 me ICate adsorp-tive capacity of the molecular sieveadsorbent. e i of a,1kah meta! e 81,02 0f e e It is well known in theart that various adsorbents, such l g li g ff g gai i e iflg i asmolecular sieves, will separate certain hydrocarbon. 9 2 3 O at oncomponents from a mixture due to a selected affinity for 35 projportmsSuch as to e rgatlo 5102 to A1203 In one or more components of themixture as for example the mixture of 3/ 1-10/ 1. This mixture is thenheld at the a mixture comprising straight chain hydrocarbons: Statediiemllerafill'l'es r at l ast an hour, and preferably branchedhydrocarbons, cyclic naphthenes and aromatics. h g ifi gg g the e i esieve fi It has been known for some time that these zeolites, both ezlre3 g e sleve matena 1S Water was e naturally-occurring and synthetic,have the property of 40 an Nate g g' H separating normal from isomericbranched chain hydrolarge Hum e f the minim l f Zee e carbons as well asfrom cyclic and aromatic admixtures. have m0leeu1ar sleve aetlvlty theablhty to ,adsorb Other types of zeolites have the ability to segregatearo- Seleenvely F i components or component Pq of matic hydrocarbonsfrom mixtures containing the same a gaseous mlxture In some cases thisSeleetwl'ty Stems as for example a mixture Comprising Straight chainfrom the fact that only molecules small enough to enter drocarbons,cyclic naphthenes, aromatics and branched the E i adselbed" Thls 1s g fgeg type hydrocarbons. These zeolites have crystalline structo esflectlfle a g p 0f f rlrlna para lris W1;fi a 11. tu-res containing alarge number of small cavities intere ar sieve i erem (my norma Para W1Connected by a number of still Smaller holes or pores enter the 5cavity. Molecular SIZG' alone, however, is not the j being ofexceptional uniform Size The pores the sole basis for selectiveadsorption. For example, the may vary in diameter from 3 to 5 Angstromunits and up selective removal of aromatic hydrocarbons by means of to12 to 15 units or more. For a particular molecular molecular Sleves froma hydrocarbon mlxture sieve material, however, the pore sizes aresubstantially Paraflins naphthenes and b e hydroca? uniform andaccordingly the material normally will be bone 15 to the much hlgher l Pthe aroma: designated by the Characteristic Size of its poreshydrocarbons have for the molecular sieve surface than The scientificand patent literature contains numerous do other me references to theadsorbing action of natural and synthetic 5 13 A molecular S16v6material's 8T6 zeolites. Among the natural zeolites having this sievevfifiilectlw? p f yp Py I11 thIS property may be mentioned chabazitesand analcite. A spect, the adsorptive characteristics of 4A. 5A. aiid13A. synthetic zeolite with molecular sieve properties is demolecularsieves are represented in the following table:

Adsorbed on Adsorbed on N 01; Adsorbed on Adsorbed 4 A. and 5 A. 5 A,but not'4'A. 4 A. or 5 A. on 13 A.

(1) Ethane (1) Propane and higher n-p'arafiiiis... (1) Isoparafiins(1)3131} hydrocarbons within gasoline 01 mg range. (2) Et ylene. (2)Butane and higher n-olefins (2) Aromatics (2) Aromatics stronglyadsorbed. (3) Propylene (3) All cyclics with 4 or more atoms (3)Diolefins strongly adsorbed.

in ring.

The present process will, therefore, separate n-paraifins from mixturesof n-parafiins and iso-paraffins and/or cyclic or aromatics by the useof A. molecular sieve. It will also separate aromatics from mixtures ofaromatics and n-paraffins and/or iso-paraffins or cyclic hydrocarbons bythe use of 13 A. molecular sieve.

As pointed out, one of the particularly attractive methods for removingnormal paraffinic hydrocarbons from a light naphtha is to contact thenaphtha with a molecular sieve adsorbent having pore diameters of 5 A.,for example. Such a sieve will adsorb straight chain, parafiinhydrocarbons but not branched chain or cyclic hydrocarbons. In acommercial process it is necessary to employ a cyclic operation, thatis, one involving an adsorption step followed by a desorption step andthen a second adsorption step, and so on. Although excellent andselective separation of normal paraflns from a naphtha can be realizedby such a procedure, one of the limiting factors is that the adsorptivecapacity of the molecular sieve decreases after a number of adsorptionand desorption cycles. The loss of sieve capacity is considered toinvolve two factors, one of them being a decrease in the saturationcapacity of the sieve, and the other that the rate of adsorptiondecreases so that for the same feed rate the sieve is less fullysaturated at the time that feed breakthrough occurs.

To some extent, the loss in capacity may be related to the methodemployed for desorbing the zeolite. Thus, they may be desorbed bypurging with an inert gas at 600 to 700 F., under a vacuum at 600 to 700F., by displacement of the adsorbed straight chain hydrocarbon by agaseous olefin such as propylene at 250 to 300 F., by raising thetemperature from an adsorption temperature of about 300 F. to adesorption temperature of about 700 F or by a combination of vacuumand-heat at about 700 F. In each type of desorption the sieve graduallyloses capacity, though not at the same rate. This temporary loss insieve capacity is due to the gradual accumulation of hydrocarbons orhydrocarbon derivatives, such as sulfur, nitrogen, or oxygen-containingcompounds, which are not desorbable and recoverable as such. The natureof these accumulated deposits varies with the feed stock, the quantityof feed treated, operating conditions, etc. Thus, the deposits may bedue to (1) polymerization or condensation of unsaturates or otherreactive components on the surface of the cavities, (2) to retention, inthe cavities, of small amounts of polar compounds present in the feed,and (3) to possible molecular rearrangements within the highly activecavity surfaces to produce branched chain or cyclic compounds which arenow too large to get out of the sieve pores, or (4) to variouscombinations of these or other related conversions.

It is the principal object of the present invention to provide animproved regenerating procedure wherein the adsorptive capacity ofmolecular sieves may be periodically restored by removing non-desorbablehydrocarbons, thereby substantially prolonging the useful life of themolecular sieves. Other and further objects and advantages of thepresent invention and the scope encompassed by the invention will beapparent from the ensuing description and from the claims. Though theremoval of carbonaceous material from spent catalysts by oxidativeregeneration is well known, such a process per se is not applicable toregenerating the crystalline metallo-aluminosilicate zeolites used inthe process of the present invention.

Because of the nature of the crystalline Zeolitic adsorbents, unless theoxidative regeneration conditions are rigidly controlled, the presenceof ordinary products of combustion, such as water, carbon dioxide andsulfur dioxide, can, at the temperatures involved in regeneration,seriously reduce the capacity of the zeolite. This reduction in capacitymay not be apparent after one or two regenerations but is sufficientlylarge to reduce the life of the zeolite to such an extent as to make itscommer- -cial application impractical.

As one factor and element in this problem, the concentration of oxygenin the regenerating gas is critical, for a given amount of carbonaceousmaterial on the zeolite determines the maximum temperature occurringduring the regeneration. Thus, oxidation of 0.5% by weight carbondeposited on the zeolite with gas containing 1.5% by volume of oxygenincreases the temperature of the zeolite from 500 to 1500 F. Exposure tothis temperature for any appreciable length of time causes a series lossof capacity. On the other hand, for the same weight of carbon on thezeolite, the use of a gas containing 0.5% by volume of oxygen increasesthe temperature of the zeolite from 500 to only 750 F.

In accordance with the present invention, the adsorbent is restoredwithout serious loss of capacity or life, by an oxidative regenerationtechnique involving critical conditions of temperature, and nature andconcentration of the oxidizing gas. Due to the nature of the adsorbent,the conditions under which the oxidative regeneration is carried outmust be maintained within specific limits in order to avoid permanentdamage to the adsorbent structure.

In brief compass, the critical conditions include a ternperature ofbetween 500 to 1000 F., a regenerative gas oxygen content of less thanabout 2%, preferably less than 1%, a water content of the regenerativegas of less than 2%, and an $0 content of this gas of less than 0.7%.Only if these stringent conditions are met can the adsorbent besuccessfully regenerated Without loss of capacity and shortening oflife.

The process may be illustrated by describing the treatment of a lightvirgin naphtha having a boiling range of about 150 to 200 F. and aresearch octane rating of about clear. A typical naphtha thuscharacterized may contain 20 to 25 percent of normal paraffinhydrocarbons, principally normal hexane, with a minor amount of normalheptane, the remaining material consisting principally of 6 and 7 carbonatom branched chain parafiins cyclic hydrocarbons. Essentially onlynormal parafiins will be adsorbed from such a naphtha on a molecularsieve of 5 A. size.

In accordance with one embodiment of the present invention in which acomplete adsorption-desorption cycle involves four stages, the heatrequired for desorbing is recovered and effectively utilized. Thecomplete cycle utilizes four reaction vessels operated in a coordinatedand integrated manner. In essence, the reaction vessels designated as A,B, C and D are operated with respect to each other and with respect tothe particular stage as follows:

Vessel Stage A B D C 1 Adsorb Heat Vacuum. 0001.

t 0 ol Adsorb.

The invention will be specifically described in conjunction with thedrawing illustrating one adaptation of the same. The reaction vesselsare designated as A, B, C and D. The operation removes normal parafl'lnsby means of 5 A. sieves. In order to simplify the description, thenecessary manifolding and valves required to change the respectivevessels from stage to stage are not shown. This piping is wellunderstood in the art.

Referring to zone A which is on adsorption, feed enters through line 1,is heated and vaporized in exchanger zone 2, and enters the bottom ofZone A at a temperature of about 250 F. Normal par afiins are adsorbedby the sieve and a normal paraff n-free product, or sievate, leaves thetop of zone A through line 3. This sievate is cooled and condensed inzone 26 and then goes to storage to be further processed as desired. Thefeed is stopped when the sieve has reached a predetermined degree ofsaturation so that essentially none of the normal hydrocarbons breakthrough with the sievate into line 3.

While zone A is on the adsorption cycle, zone B is on heating or thedesorption cycle. Heating is accomplished by circulating desorbate vapordownwardly through the sieve zone B, through compressor 5, exchanger 6,exchanger or furnace 10, and'back to the top of the sieve zone. Thedesorbate vapor used enters through line 19 either from another sievezone or from storage. Product desorbate is removed by means of line 17and handled as desired. The recycled desorbate vapor is heated inexchanger 6 to a temperature in the range from about 350 to 550 F. as,for example, of about 450 F. and

further heated in zone 10, which may be a heat exchanger or furnace, toa temperature in the range of 600 to 700 F. as, for example, of about650 F. Thus, the sieve temperature at the inlet of zone B Will start torise to a higher temperature. As the heating cycle progresses, the sieveat the inlet of zone B will be about 650 F. and that at the outlet willbe at a temperature somewhere between 250 and 650 F. A heat front ofabout 650 F., under the conditions specified, will move downwardly fromthe top of the sieve zone to the bottom or outlet. When the temperatureof the gas leaving vessel B is about 400 to 500 F. as, for example,about 450 F., the heating stage is completed. Zone B may be operatedduring heating at approximately the same pressure as Zone A. However, itis preferred that the pressure in Zone B be in the range from 30 to 50p.s.i.g. This will permit optimum sizing of the compressor.

At the end of the heating cycle, vessel B is swung to the vacuum stageillustrated by vessel D. Vacuum is imposed on vessel D by opening itthrough line 27 to condenser 16. Condenser 16 is operated at such atemperature as to give the desired vacuum in zone D. This may vary from400 mm. mercury absolute to 50 mm. mercury absolute. The preferredvacuum will depend upon the condensing temperature available incondenser 16. From economic considerations, the preferred condensingtemperature is about 30 to 60 F. With a C to C hydrocarbon feed thiswill give a vacuum in zone D of approximately 150 mm. mercury absolute.Product desorbate is removed from the condenser 16 by means of line 14.

While zone A is on adsorption, zone B is on heating and zone D onvacuum, zone C is on the cooling cycle. Cooling is accomplished bycirculating sievate, that is, normal-free vapor by means of pump orcompressor 9 through line 8 upwards through the sieve zone, then throughexchanger 6 and exchanger 12. At the start of the cooling phase, aportion of the sievate from zone A is injected into this circuit bymeans of line 11 and compressor 9. This sievate, or rafilnate, vapor isintroduced into bottom of zone C at a temperature in the range of about200 to 300 R, such as approximately 250 F., and contacts the sievematerial which is at about 650 F. The stream leaves the top of zone C atapproximately 650 F., is cooled in exchanger 6*to approximately 450 F.,and in exchanger 12to approximately 250 F., at which point it isrecycled back'to the bottom of the sieve zone C through compressor9 andline 8'. Under these conditions a cooling front'will-move upwardlythrough vessel C and at an intermediate point in the cooling stage thetemperature at the bottom of vessel C will be about 250 F. and thetemperature at the top of vessel C will be between 250 and 650 F.Cooling will be terminated when the outlet gas'from the-top of zone Creaches a temperature in the range of 400 to 500 F. such asapproximately 450 Heat is exchanged between the sieve on cooling in zoneC and the sieve on heating in zone B by means of exchanger 6. Under theconditions of this invention, the

6 heat requirements are minimized by the use of this exchanger. Only asmall amount of incremental heat must be supplied through exchanger 10and likewise only a minimum removed through exchanger 12.

Pressure in zone C may vary but the preferred pres sure is between 30and 50 p.s.i.g. in order to minimize compressor requirements.

It is also to be understood that further heat economy can be obtained inthis process by using incoming feed as the coolant in exchangers 16 and12.

Typical operating conditions for the treatment of a pent-ane-hexanefraction are as follows:

Time, min. 5

Amount, gm./ gm. sieve 1.4 Temp., F. (end) 380 Vacuum:

Final press., mm. Hg. abs Time, min. 12 Temp., F. (end) 565 Cooling:

Pressure, p.s.i.g 30 Time, min. 15 Inlet temp, F. 260 Amount, gm./ 100gm. sieve 50 Temp., F. (end) 365 It is to be realized, as pointed outheretofore, that operating conditions may be varied, depending upon thecharacter of the feed stock and other variables. While the coolingstage, as described, utilized sievate, it is to be understood that othercooling media such as hydrogen, methane, light hydrocarbon and the like,may also be used.

After a number of adsorption and desorption cycles when itis determinedthat the adsorptive capacity of the molecular sieves has beenappreciably reduced, the molecular sieve in zone A is subjected to theregeneration step of the present invention. As previously stated, thisstep consists in removing all desorbable hydrocarbons, raisingthetemperature of the bed or a portion of thebed to a range of 500 to 1000F., and passing therein an oxygen-containing gas through line 1. Ingeneral, temperatures above 1000 F. should be avoided because 'of dangerof damage to the sieve. The temperature may be suitably raised by firstpassing through the bed a hot purge gas suchas fiue gas or nitrogen bymeans of line 1, the gas leaving the tower through line 3. When thedesired temperature has been reached, the flow of purge gas isdiscontinued and the regenerating gas admitted. In all events,temperature is carefully controlled to avoid heating the sieve aboveabout 1000 F. for any significant period of time.

An important feature of the present invention is the removal of alldesorbable hydrocarbons preceding the introduction of oxygen. Thiscritical step is necessary to minimize the-generation of hightemperature "steam in the sieve which results from hydrocarbonoxidation. In this respect the oxidation of the non-desorbable residualhydrocarbons from the sieve differs from the usual type of catalystreactivation, such as cracking catalyst, where the carbonaceous depositsare almost entirely coke, the percentage of hydrogen being very low.

At the end of the cycle preceding the oxidation treat, the sievecontains small amounts of normal paraffins. Hence, the particular meansemployed for removing desorbable hydrocarbons from the sieve precedingoxidation will usually depend on the type of cyclical operatingprocedure being employed. Therefore, various combinations of purging,evacuation, and low temperature steaming may satisfactorily be employed.Of the preferred methods are (l) steaming at temperatures below about600 to 700 F., and (2) purging with an inert gas, such as nitrogen,methane, etc., during the time the sieve bed is being heated to thehigher level preparatory to the oxidation step. As already indicated,these gases may be preheated and used to elevate the bed temperature.

The resistance of the A. type sieve to degradation by high temperatureis illustrated by the following data obtained at calcinationtemperatures of 850 to 1500 F. in a dry atmosphere.

Calcination Conditions Absorption Capacity for n-heptano,

(id/gram The 5 A. sieve appears stable indefinitely at 950 F. in theabsence of moisture and for a reasonable length of der conditions whichcause the partial or more or less complete oxidation of thenon-desorbable residual hydrocarbons or residues. Temperatures whichmust be held in the sieve bed to maintain combustion will vary betweenabout 650 and 1200 F., preferably below about 1000 F. The oxygen contentof the gas is less than 2%, preferably less than 1%.

The method for carrying out the oxidation in fixedbeds consists ofburning in a wave-front procedure so that the temperature of the entire-bed is not elevated. In this procedure the initial temperature of thesieve and oxygen-containing gas are such that a burning-front isestablished at the gas inlet to the sieve bed. Thus,

the initial, inlet bed temperature should be about 600 to 900 F. Thecombustion products, inert gas, steam, and any desorbed, partiallyoxidized hydrocarbons, are driven ahead of the burning front intoportions of the sieve bed at lower temperature levels where the steamdoes no harm. The burning front may be initiated by (l) preheating theoxygen-containing gas, (2) use of oxidation promoters in the gas, suchas oxides of nitrogen, and (3) oxidation promoters on the sieve, such asCu, Mn, Cr, Fe, etc., introduced either by impregnation or by ionexchange with the sieve, or by other suitable 5 means. A furtheradvantage to this procedure will result from the use of a dryoxygen-containing gas in that the clean sieve following the burningfront will be simultaneously dried in the short time held at theelevated temperature.

An excellent method of carrying out the burning is to do it in threephases, in order to control sieve bed temperatures. The first phaseconsists of purging the sieve with inert gas at high temperature. In thesecond phase, oxygen is blended with the inert purge in lowconcentrations, and the third phase consists of passing preheated airover the sieve bed. The following conditions may be used in burning.

time at 1300 F. It is destroyed slowly at 1400 F. and quickly at 1500 F.Prolonged steaming at 950 F. and above causes losses in adsorptivecapacity and adsorption rate, although the X-ray crystal pattern was notchanged. Illustrative data are shown in the following tabulation forvarious 5 A. modifications.

STEAM STABILITY OF VARIOUS 5A. MODIFICATIONS 950 F., 1 ATMOSPHERE STEAMn-Heptane Adsorption Meta. Form Time, X-ray of Sieve hours Capacity,Relative Examination ce./g. rate 0 0. 19 1 5A. Pattern. 113 19 18 NoChange.

0 l6 1 5A. Pattern. 162 .13 3 No Change.

0 l8 1 5A. Pattern. 103 14 5 No Change.

Steam deactivated 5 A. sieves can in some instances be restored to theiroriginal adsorption rates by low temperature steaming, such as thatemployed in the hydrocarbon desorption step described above. However,losses in adsorptive capacity are usually more permanent and are to beavoided.

Following the removal of desorbable hydrocarbons, an oxygen-containinggas is introduced into the sieve bed un- The most critical variable isthe concentration of oxygen in the inlet gas during the second phase ofregeneration. An 0 concentration of 0.75% has been found to limit sievebed temperature rise to about 250 F. (starting at 700 F. and peaking at950 F.)

The following examples illustrate the benefits obtained by the presentinvention:

EXAMPLE 1 In a vapor phase cyclic operation of the type described above,n-heptane was adsorbed from an toluene- 20% n-heptane mixture by analumino-silicate of the 5A. type at 240 F. and then desorbed withpropylene at the same temperature. With the fresh sieve the propylenedesorbed 87% of the adsorbed n-heptane. After 14 cycles, the capacity ofthe zeolites had decreased to about 75%. The latter was then heated inan air stream for 2 hours at 850 F. Complete reactivation was effected.

A 5 Angstrom crystalline calcium-sodium aluminosilicate discharged froma pilot plant in which normal parafiins were removed from straight runnaphtha was regenerated by burning in an air stream, and the capacityfor n-hexane' determined. The burning step employed an air rate of 0.05cubic feet per minute, equivalent to 0.3 gram oxygen. For comparison,results obtained by reg. generating (1) with vacuum and (2) with steamat 250 F. are included. High temperature steam cannot be employed as itdestroys the sieve structure.

Adsorptive Weight Surface Pore Capacity, Percent Area, Vol.,

g./100 g. Carbon mJ/g. ce./g.

Fresh Sieve 10. 1 0.00 497 0.25 Used, desorbed at 700 F.,

4 mm 8.1 2. 3 408 0. 19 Used, desorbed at 850 F.,

1 mm 8. 6- 2. Used, desorbed at 1,000 F.,

1 mm 8. 6 1. 3 434 0.22 Used, burned at 850 F.,

in air 10. 0 0. 2 471 0. 22 Used, burned at 1,000 F.,

in air 10. 2 0. l 467 0. 23 Used, steamed at 250 F 7. 3 1. 3

These data show clearly the superiority of the regeneration technique ofthe present invention. Burning the carbon off in an air streamregenerated the sieve completely, while vacuum at high temperatures didnot give method for removing S0 from flue gas which may contain, onrecycle, up to 2% of water resulting from combustion of carbonaceousdeposits, is to contact the flue gas with silica gel. It is particularlyunexpected that this reagent is eflective for S0 removal inasmuch as thesmall amount of water present might be expected to be preferablyadsorbed. It has been found that the capacity of the gel for S0 from thewet flue gas is substantially equal to the capacity of the gel for S0from a dry gas. These results are shown in the two tables below:

S0 IN FLUE GAS DAMAGES MOLECULAR SIEVE.[Temperature, 1,000 F.]

Cumulative Capacity Loss (g./100 g.) at Indicated Time Intervals GasComposition (V 01. Percent) (Time in Hours) S02 CAN BE REMOVED FROM WETFLUE GAS BY SILICA GEL Outlet Gas From Silica Gel Bed 0.006% S02 H2O notmeasured Inlet Gas to Silica Gel Bed Measured Capacity of Silica Gel forS0 (T0 S02 Breakthrough) 0.6 gm./100 gm-.

Published Capacity of Silica Gel for Dry S0 (Saturation Capacity) 0.5gm./100 gm.

Perry, J. H.: Chemical Engineers Handbook, 3rd edition, Fig. 52, p. 912,McGraw-Hill Book 00., Inc., New York 1950.

the same extent of improvement, and steaming actually decreased theabsorptive capacity- EXAMPLE 2 As pointed out above, it is essential toemploy a regeneration gas which is substantially dry. Flue gases andother regeneration gases low in oxygen, containing more than 2% amountof water, either are not suitable or must be dried prior to use inaccordance with the present invention. Particularly since the recyclingof flue gas is desirable, and since water is formed as a result of thecombustion of carbonaceous deposits, means of removing water from therecycle flue gas is important. Tests were carried out in bench scaleunits using 25 grams of sieves. Synthetic flue gases containing variousamounts of nitrogen, carbon dioxide and water were employed. The sievewas exposed to these gases at 900 F. and 1000 F. for at least 100 hoursper test. The capacity of the zeolite was measured before and afterexposure by first desorbing at 850 F. and 0.2 mm. Hg. abs. and thenadsorbing n-hexane at 250 F. and 150 mm. Hg. abs. The results are shownin the table below:

Cumulative Capacity Loss Gas Composition Temp. (g./l00 g.) at IndicatedTime (V 01. Percent) F.) Intervals (Time ln-Hours) Nitrogen Plus- CO1900 0.1 0.2 0.3 12% C02, 2% H O" 900 0. 1 0.3

12% C02, 2% H2O 1,000 0.3 0.5 0.9 1.2 17% C0 7% H20 900 2.1 2.9 3.4

These data, depicting cumulative capacity loss as a function of the timefor the various test conditions, show that deactivation and capacityloss become serious when the water content was increased from 2% to 7%.

EXAMPLE 3 These data show that S0 is definitelydetrimental to sievecapacity. Nearly 30% of the sieve capacity was lost after hours.Furthermore, the silica gel treating step is highly useful in solvingthis problem.

As has been pointed out, the oxygen concentration used in regenerationis controlled by two factors, namely (a) the resulting temperature, and(b) the resulting concentration of damaging products of combustion,namely water and S0 Because of the stoichiometry of combustion of thecarbonaceous deposit from the molecular sieve adsorbent used inhydrocarbon separations, the oxygen concentration must be less than 3%to hold the water concentration to 2% or less, in accordance with theequation:

centrations below 0.5%, therefore, does not result in excessivetemperatures. Above 0.5% deposit, however, dilute air must be used tokeep the temperature within bounds. The oxygen concentration must beless than about 1%, preferably less than 0.7%. The above figures arebased on theoretical calculations for an adiabatic regeneration.

Thus, it is highly undesirable to employ an oxygen content greater than3% because of the resulting water formation and this, regardless of theamount of carbon on the sieve. Secondly, if more than 0.5 wt. percent ofthe deposit is on the sieve, the amount of oxygen must be limited to0.7% to keep the temperature at an operable limit. Accordingly, 3%oxygen may be employed if the deposit is less than 0.5 wt. percent and0.7 wt. percent is employed if the deposit is greater than 0.5 wt.percent. Were only carbon deposited on the adsorbent, undiluted airmight be used provided the carbon was present to the extent of less than0.5 wt. percent. How- EFFECT ON MOLECULAR STEVE OF OXYGEN CONCEN-TRATION AND WEIGHT PERCENT CARBON DEPOSIT DURING REGENERATION Initialbed temperature, 700 F.

Maximum tolerable temperature, 1,000 F.

Maximum tolerable H20, 2%.

Carbonaceons deposit has atomic ratio of hydrogen to carbon 012/1 (CH1)Wt. Percent Deposit. 0.1 0. 5 1.0

Vol. Percent in Inlet Gas 0.5 3 21 0.5 3 21 0.5 3 21 Temp. of Bed, F.790 758 753 940 1,200 970 870 2,700 1,300 Percent H2O in Product Gas 0.32 14 0.3 2 14 0. 3 2 14 The process of the present invention may bemodified in many details without departing from its spirit. Thus, it hasalso been found that, in addition to the relatively slow loss incapacity described above, there is also experienced a much more rapidloss in cycle capacity associated primarily with the use of propylene asdesorbent. These losses, however, though caused by the presence ofdeposits on the sieves, may be satisfactorily removed by vacuumregeneration at about 700 F. This type regeneration may be carried outat about every 200 cycles depending on the conditions used. Theoxidative regeneration technique is satisfactorily carried out inconjunction with the vacuum regeneration. That is, those impurities thataccumulate either on or in the sieves that cannot satisfactorily beremoved by vacuum are then removed by periodic oxidative regeneration.

Furthermore, not only may the spent 5 Angstrom zeolites be regeneratedin this manner, but the same or similar technique may also be applied tozeolites having smaller or larger uniform pores, from 3 Angstroms to 15Angstroms. The and 13 Angstrom zeolites have the capacity of separatingisomeric branched and cyclic hydrocarbons, have catalytic properties,and also tend to become deactivated.

In a still further embodiment of the invention, the spent sieve may beremoved from the adsor-ber and conveyed by a moving screen or the likeover a burner zone.

Advantage may also be taken of the catalytic cracking characteristics ofthe sieve to regenerate it. This is particularly useful when not toomuch carbon as such is deposited on the zeolite. Under these conditions,a temperature of about 550 to 900 F. is imposed upon the bed of usedsieves at a pressure of one atmosphere and less. A small amount ofoxygen or other promoter is then added as a cracking promoter, and asmall amount of steam to suppress carbon formation. Thereafter, residualcarbon may be removed in the manner previously described.

I claim:

1. A process for regenerating a bed of crystalline zeolitic molecularsieve by combustion of non-desorbable carbonaceous deposits accumulatedthereon which comprises the steps of:

(1) raising the temperature of said bed to within the range of about 500to 1000 F.;

(2) contacting said bed with high temperature regenerating gascontaining less than about 1% oxygen to substantially combust saidcarbonaceous deposits;

. and

(3) thereafter increasing the oxygen content of said regenerating gas upto 21% oxygen to combust residual carbonaceous deposits;

wherein the oxygen content of said regenerating gas is controlled tomaintain a combustion temperature below the degradation temperature ofsaid molecular sieve and to hold the water concentration to below about2%.

2. The process according to claim 1, wherein the S0 content of saidregenerating gas is held below about 0.7%.

3. The process according to claim 1, wherein the weight percent of saidcarbonaceous deposits, based on the weight of said molecular sieve, isabove about 0.5 in Step 2 and below about 0.5 in Step 3.

4. The process according to claim 1, wherein the oxygen content in Step2 is maintained at about 0.7%.

5. The process according to claim 1, wherein Step 1 is accomplished bypurging said bed with inert gas at high temperature.

6. A process for periodically restoring the adsorptive capacity of acrystalline zeolitic molecular sieve having non-desorbable carbonaceousdeposits accumulated thereon by a three-stage regenerative treatmentwhich comprises (1) purging with an inert gas at a temperature withinthe range of about 500 to 1000 F.; (2) contacting with a hightemperature oxygen-containing gas within said temperature range, saidgas containing less than about 2% oxygen, less than about 2% H 0 andless than about 0.7% S0 and (3) contacting with high temperature airwithin said temperature range; to'thereby combust said carbonaceousdeposits.

7. In a process wherein hydrocarbons are separated by adsorption fromtheir mixtures by contacting such mixtures with crystalline zeoliticmolecular sieves at a temperature in the range of from about 200 to 500F. and adsorbed constituents selectively desorbed by applying heatthereto in a subsequent desorption step, and wherein non-desor-bablecarbonaceous deposits gradually accumulate on said zeolites, theimprovement which comprises treating a flue gas so as to form anoxygen-containing regenerating gas containing less than 2% 0 less than2% H 0 and less than 0.7% S0 and periodically restoring the adsorptivecapacity of said adsorbent after a selected number ofadsorption-desorption cycles and at least 0.5% by weight of saidcarbonaceous deposits accumulate on said adsorbent by subjecting theadsorbent to a temperature of from 500 to 1000 F. in the presence ofsaid thus formed oxygen-containing regenerating gas.

8. The process of claim 7 wherein said adsorbent is substantiallycompletely freed of desorbable hydrocarbons prior to said regenerationstep.

9. The process of claim 7 wherein said regenerating gas contains about0.7% oxygen.

10. The process of claim 7 wherein said sieve adsorbent is subjected toa three-stage regenerative treatment wherein in the first stage it ispurged with an inert gas at elevated temperature, in a second stage witha gas containing less than 2% oxygen, and in a third stage with air.

11. The process of claim 7 wherein said treatment of flue gas isaccomplished by contacting said flue gas with silica gel.

12. A process for removing non-desorbable carbonaceous deposits from abed of crystalline zeolitic molecular sieve, said bed having a gas inletand a gas outlet, which process comprises:

(1) raising the temperature of at least the inlet portion of saidmolecular sieve bed to within the range of about 500 to 1000 F.;

(2) thereafter flowing a stream of high temperature regenerating gascontaining less than about 1% oxygen through said molecular sieve bedfrom said gas inlet to said gas outlet to substantially combust saidcarbonaceous deposits; and

(3) thereafter increasing the oxygen content of said regenerating gasstream to a higher oxygen concentration to combust the residualcarbonaceous deposits;

wherein the oxygen content of said regenerating gas stream is controlledto maintain a combustion temperature below the degradation temperatureof said molecular 13 14 sieve and to hold the water concentration ofsaid regen- FOREIGN PATENTS crating gas stream at said gas outlet tobelow about 2%. 777,232 6/1957 Great BTitam References Cited DANIEL E.WYMAN, Primary Examiner.

UNITED STATES PATENTS 3,030,431 4/1962 Mattox et a1. 260-676 3,069,36212/1962 Mays et a1. 252-419 5 c. F. DEES, L. G. XIARHOS, AssistantExaminers.

